In 3rd Generation Partnership Project (3GPP) Release 12, dual connectivity (DC) is being standardized, wherein a user equipment (UE) can be simultaneously connected to a master evolved Node B (MeNB) of a macrocell and a secondary eNB (SeNB) of a picocell. The primary motivation of DC is data boosting by using a large number of picocells in hotspots. However, this leads to highly decreased mobility robustness because the UE needs to perform frequent handovers due to the small size of the picocells. In order to overcome this issue, the architecture of DC is used in which mobility is handled by an umbrella macrocell.
In this new architecture of DC, the UE may be allocated an uplink data channel in both an MeNB and an SeNB. It is to be noted that an MeNB is considered a controller for many control plane functionalities, but a data plane is largely independently performed in each of an MeNB and an SeNB. The primary reason is that an inter-eNB communication delay is much higher than the typical scheduling interval. The inter-eNB communication incurs a delay of 2 to 6 ms which is magnitudes higher than the scheduling delay of 1 ms. Accordingly, an eNB performs its own scheduling. This can also be considered as inter-eNB carrier aggregation (CA).
Further, in DC, it is agreed that the cells of an MeNB are referred to as a primary cell group, and one cell of an MeNB is treated as a primary cell (Pcell). The cells of an SeNB are referred to as a secondary cell group, and one cell of an SeNB is treated as a primary cell of the secondary cell group and is referred to as a primary secondary cell (pScell).
In such a DC system, the cells of an MeNB and an SeNB may not be synchronized at a physical layer (PHY) level. Further, a system frame number (SFN) may not be synchronized between the cells of the MeNB and the SeNB. In such a system, even when the system information of the SeNB cells can be provided to a UE before the addition of a cell via the MeNB Pcell, the UE will be unable to communicate with the “new” cell of the SeNB immediately because it will not be SFN synchronized with the “new” cell. In this case, the UE will be unaware of the SFN corresponding to a radio frame in the new cell.
The SFN is included in a master information block (MIB) broadcasted by a cell. The MIB is, for example, broadcasted every 40 ms. The broadcasted SFN in the MIB corresponds to the frame in which the MIB is received. Based on the reception of the MIB, the UE will become SFN synchronized with the new cell. Further, it is to be noted that although the MIB of the new cell can also be provided to the UE like a system information block (SIB) via the MeNB Pcell, but the SFN information cannot be provided because there will be a delay between information transfer between the SeNB and MeNB and thus the provided SFN will not correspond to the correct radio frame, as a result of which the UE will be unable to communicate with the new cell.
If the SFN offset between an MeNB secondary cell (Scell) and an SeNB Scell is known to the MeNB, then the MeNB can signal the offset to the UE and then UE can become SFN synchronized with the SeNB Scell because it will be already aware of the SFN in the MeNB Pcell. However, it cannot be guaranteed that the MeNB can always know the offset between the MeNB and the SeNB. This may occur, for example, when the MeNB and the SeNB belong to different operators.
In such a system, when the MeNB wants to add a new cell in the secondary cell group, it can signal the new cell addition to the UE, but the SeNB cannot activate the new cell immediately because the UE may not have decoded the MIB which contains the SFN of the SeNB. The SeNB may need to give a sufficient time delay prior to the activation of the new cell so that the UE can decode the MIB directly broadcasted from the new cell. This delays the activation of the new cell.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.